未知宽高或已知宽高的水平、垂直居中

// 1
.wraper {
  position: relative;
  .box {
    position: absolute;
    top: 50%;
    left: 50%;
    width: 100px;
    height: 100px;
    margin: -50px 0 0 -50px;
  }
}

// 2
.wraper {
  position: relative;
  .box {
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
  }
}

// 3
.wraper {
  .box {
    display: flex;
    justify-content:center;
    align-items: center;
    height: 100px;
  }
}

// 4
.wraper {
  display: table;
  .box {
    display: table-cell;
    vertical-align: middle;
  }
}